The invention relates to a continuous countercurrent ion exchange process employing a liquid-solids contactor.
The application of ion exchange techniques for the separation of substances on an industrial scale has resulted in the development of continuous processes whereby a feed liquor containing the substance of interest is uninterruptedly made to flow through a bed of ion exchange resin particles where the substance was absorbed onto the particles. In order to maintain the efficiency of the process, means have been devised to remove loaded resin particles from the absorption bed to an elution bed where the substance of interest is removed from the particles. In a continuous process, movement of particles from the absorption bed to the elution bed and back is required. U.S. Pat. Nos. 4,018,677 and 4,035,292 in the name of the present applicant, show processes wherein intermittently a batch of loaded particles is removed from an absorption column and is transferred to an elution column.
When designing an absorption process for incorporation into a continuous countercurrent system, the simplest approach is to provide a suitable chamber with a single bed of ion exchange resin particles. One method of bringing the feed liquor into contact with the resin particles is to maintain the resin bed in a fluidized state and to flow the feed liquors vertically upwardly through the fluidized bed. While the ions of interest will be absorbed onto the resin particles when contacting a feed liquor, with such a fluidized resin bed there is a tendency for the particles to classify according to particle size and density. This will eventually lead to back-mixing of the resin particles, i.e., a mixing of loaded particles with unloaded particles. This back-mixing is undesirable because in such a fluid-solid contactor it results in entrapment of loaded resin particles in the upper region of the fluidized bed thereby greatly reducing the efficiency of the separation process.
Various methods have been described to overcome these problems. The applicant has developed a multichambered contactor as disclosed in U.S. Pat. No. 4,035,292, which has proved to display a high degree of utility in overcoming these problems.
Although the continuous contactor disclosed in U.S. Pat. No. 4,035,292 is capable of handling feed solutions containing higher levels of suspended solids than prior fixed bed columns, in order to achieve transfer of batches of resin particles between vertically adjacent compartments of the contactor column, normally reliance is placed on withdrawing liquid from a lower compartment and recirculating this liquid through a pump to the adjacent upper compartment, whereby a net downflow of liquid between the two compartments can be achieved which carries the particles down with it. The liquid is withdrawn from the lower compartment through a screen located in the lower compartment in order to prevent resin particles from being drawn into the pump. This column is not completely satisfactory for use with highly turbid feed liquors having levels of suspended solids up to 20 to 30% as there is a risk of the screens within the column becoming blocked and access to these screens for cleaning purposes cannot, in most instances, be gained without shutting down the column, and, moreover, in the event of a pump failure there is risk of the suspended solids settling and clogging the conduits that interconnect the compartments externally of the columns.
The present invention relates to the use of a fluidized resin bed for absorption of a substance from a feed solution whereby the resin particles appreciably increase in density as they become loaded thereby migrating toward the lower region of the chamber. This effect can be observed in a number of hydrometallurgical applications, for example, the extraction of uranium from a pregnant leach liquor.
Because the resin particles increase in density as they are loaded, and thus, classify toward the bottom of the fluidized bed, in the process of the present invention, back-mixing is considerably reduced and loaded particles can be collected continuously from the bottom of the column. It has been found that a considerable increase in absorption column efficiency is obtained by providing a small fluidized bed above the main bed to remove substantially all residual ions of interest from the out-flowing solution.